CHORDATA 
CLASS UROCHORDA or TUNICATA 
HE most conspicuous animals of this class are the ascidians, 
which are common objects on rocky coasts. The simple 
ascidians are peculiar leathery, sac-shaped bodies which send out 
jets of water when touched. This habit gives them the common 
name of “sea-squirts.”. Some are highly colored, especially 
those of Southern waters; others are somber, unattractive bodies, 
often growing in masses. The compound ascidians are gelatinous 
colonies, sometimes forming thin incrustations, sometimes jelly- 
like masses, on seaweeds, shells, etc. This class comprises also 
the beautiful Salpa, a genus of free-swimming animals having 
transparent bodies encircled by rings of muscular bands, and in 
one stage forming chains of attached organisms which swim on 
the surface of the sea and of bays. 
The tunicates are by some authors classed with the vertebrate ani- 
mals because in the larval stage they have a notochord ; this disap- 
pears, however, in the adult form, and the animals are considered 
degenerates. The tunicates are interesting to biologists from the 
remarkable changes they undergo in their life-history, and (in some 
genera) the marked phase of alternation of generation. One of 
their curious anatomical features is the blood-vascular system. 
The circulation is propelled by wave-like contractions of the 
heart, which, after forcing the blood one way for a time, stops 
and reverses the blood-current. The tunicates are widely dis- 
tributed, and occur at all depths. (Plate LXXXV.) 
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